Er. Vivek Abhyankar, Deputy GM (Design), L&T, Mumbai & Dr. N. Subramanian, Consulting Structural Engineer, Gaithersburg, MD, USA
There are several failures of buildings and bridges and other constructed facilities; it is important to learn from these failures so that such catastrophes do not occur in the future (Feld and Carper, 1996, Levy and Salvadori, 2002, Raikar, 1987, and Salvadori, 1990). Failure of buildings due to construction of a deep trench in the adjoining plot is not a new phenomenon, although it is undesirable. Recently, a building collapsed in Bengaluru into the adjoining trench which had been dug for a new construction, to replace an iconic theatre building. Similar fatal collapses have occurred in various other parts of India and across the world.
Manmade trenches and slopes or natural landslides are equally alarming and devastating for existing buildings. A boundary wall of a multistory building in Wadala (Mumbai) collapsed due to a landslide in June 2018. There was the pile foundation failure followed by the complete collapse of a G+13 building in Shanghai (Fig 1 & 2).
Fundamental Behavior of Soil and the Phenomenon of Building Collapse Due to Adjoining Excavation
Earth or soil has a defined safe bearing capacity, which is derived from the adhesion between soil particles and their shear strength based on its Cohesion C and angle of internal friction φ. Another important criterion for deciding the SBC of soil strata is the settlement. The shear strength and settlement are dependent on the Cohesion C, angle of internal friction φ, void ratio and the internal pore water pressure. A typical three phase diagram of soil is indicated in fig. 3 (ref. NPTEL courses). When the air voids get filled with water during rainy season or for other reasons, then there is buoyancy as well as loss in shear strength of soil.
When the soil is loaded with surcharge on the top, the behavior differs as per the particle size (sand, silt or clay); but primarily the voids may get compressed and there will be a lateral movement to maintain the volumetric stability (Poison’s effect). This generates the lateral earth pressure; and depending on the freedom of movement it could be further classified as – ‘active’, ‘passive’ or ‘at rest’ earth pressure.
Now let us image a building adjoining to which a trench is excavated. Excavation of trench leads to imbalance of forces in the soil particles on the vertical face of the trench. If the shear strength (Q = C + σ tanφ) is higher than that of the unbalanced lateral force (ka.γ.H) then the vertical cut is stable, for example, often we find several workers / plumber / electrical or telephone line repair workers dig small size trenches adjacent to the building and fill back after the repair work is over, but the building does not collapse. But if the trench size is enormously large and deep (extending below the foundation level, or ground water table or if the sides of the trench is un-protected or if there is seismic activity or heavy rains, etc) then there is always a danger of collapse of the building.
The excavation contractor should always consult qualified and experienced structural /geotechnical engineer to ‘maintain’ proper equilibrium of forces on the cut earth face. Depending on – (1) the type of strata (soil, silt, clay, gravel, rock, mixed strata), (2) depth of the excavation, (3) extent of pore water pressure, (4) extent of surface surcharge, i.e. weight of building, (5) seismic zone, (6) duration of exposed cut, and (7) distance of the cut from existing building foundations, a suitable treatment should be provided, as described further in this paper (See also Yu, S., and Geng, 2009).
In urban areas, the cost of every foot of a land is very high, hence we notice very congested development of plots almost touching each other (Fig. 4 and 5 show typical structures in congested areas of a few metro cities in India).
Due to the high land cost in cities, the land owners often explore the option of ‘underground basements’. When deep excavations are done very close to the existing buildings, there will be large lateral deflection of the retaining wall, which will lead to movement of ground below the foundation of existing building and subsequent cracks and tilting of the building (Fig. 6)
1990 Excavation Collapse at 14th and H Streets, N.W. Washington, D.C.
An open excavation, 150 ft. x 208 ft. by 47 ft. deep for a 12-story office building with four levels of parking at 14th and H Streets, N.W., Washington, D.C., collapsed on November 19, 1990 at approximately 8:30 p.m (Fig 7). Construction had stopped for the day at the time of this accident. Hence no injury or death was reported, though there was significant potential for casualties.
The National Office of the Occupational Safety and Health Administration (OSHA) investigation began soon after the accident and included observation of the collapsed structure and geotechnical and structural analysis of the excavation support system. Based on this investigation, OSHA concluded that:
- The internal support system erected at the construction site, was not capable of resisting the soil load which could be reasonably expected. Certain members and their connections were not adequately designed to suit the field conditions for the expected loads.
- The spacer beams (outlooker beams) between the wale and the soldier beams and their connections and the sloping wale were most highly stressed and contributed to the collapse.
On June 27, 2009 at 5.30 am, an unoccupied 13-storey block of flat building, still under construction, at Lianhuanan Road in the Minhang district of Shanghai city, China toppled over and ended up lying on its side in a muddy construction field (See Fig. 1 and 8). One worker was killed. The investigation report concluded that this collapse was caused by earth, excavated along the building on one side with a depth of 4.6 m, for an underground car park, and piled up to heights of up to 10 m on the other side of the structure, as seen in Fig. 1 and 8 (Subramanian, 2009, and Wang et al., 2017). The weight of overburden earth created a pressure differential, eventually weakening the foundations and causing them to fail. This situation might have been aggravated by several days of heavy rain leading up to the collapse.
2018 Landslide near a multistory building in Wadala, Mumbai
In June 2018, the compound wall of Lloyd Estate in Wadala, Mumbai collapsed, leading to several cars caving into the neighbouring under-construction site of Dosti Realty, loose soil from the spot slid away, triggering fears of another landslide at the spot [see Fig. 2(b)].
2020 Collapse of two buildings into adjacent trench in Bengaluru
On July 29, 2020, right behind what was once the iconic Kapali Cinema Theatre in the Majestic area, Bengaluru, two leaning buildings (three-storey hotel and a four-storey PG accommodation), collapsed but fortunately there were no casualties as the police had evacuated all the inhabitants. The building contractor had dug a 65-foot-deep trench (to build four basements, a ground floor and five floors on a 50,000 sq foot plot), without putting in place adequate support. This apparently caused the two adjacent buildings to collapse into the trench. The Majestic area once had ponds and lakes and it is speculated that the raising ground water might have contributed to the weakening of the soil and subsequent collapse of the buildings into the trench, whose three sides were only soil nailed and without constructing any retaining structure (see Fig. 9).
Treatments to Avoid Such Failures
As mentioned earlier, the treatment of exposed surfaces depend on (1) the type of strata (soil, silt, clay, gravel, rock, mixed strata), (2) depth of the excavation, (3) extent of pore water pressure, (4) extent of surface surcharge, i.e. weight of building, (5) seismic zone, (6) duration of exposed cut, and (7) distance of the cut from existing building foundations.
Following are the popular treatments:
1. Bare Exposed face – When the depth of cut is fairly shallow, the loads/surcharge is lesser in magnitude or located far away, the soil is sufficiently strong (hard rock or weathered rock) and the duration of exposed cut is short (one or two weeks) then the slope need not be treated after consultation with the competent authority.
2. Exposed face with shotcrete – When all the conditions are same as case one but there is minor seepage from crevices in the strata or fear of falling soil particles from the exposed face pockets, then same can be addressed using shotcrete. In some cases the shotcrete could be supported with a wire mesh. Such slopes can fairly work for longer time and depths up to 2 to 3m. But care should be taken that the seepage is not pronounced; otherwise it may endanger the bonding of shotcrete with the strata.
3. Soldier piles and timber planks lagging – When the soil is strong but with higher loads, and shallow depth of the cut, solider piles and timber planks lagging as shown in Fig. 10.
4. Cantilever Sheet pile protected face – When deeper excavation (say up to 5 to 6m depth) is carried out to retain comparatively heavy surcharge (say up to G+5 storey building with foundation depth deeper than the excavation), with fairly good soil strata and for a longer duration of pit but without any seepage estimated from the base of the pit and when stringent limit on the soil settlement/movements at the tip of the wall, then cantilever sheet pile wall is a good solution.
Steel sheet piles can also be used to retain railway/road embankments while doing excavation closer to the tracks. Steel sheet piles are slightly costlier and required to be driven with the vibro-hammers. They are reusable; a greater number of repetitions can recover the initial investment cost. But while driving the piles adjacent to buildings with fragile glass façade, care shall be taken to reduce the vibrations generated during the pile driving operations (otherwise, breaking of glass panels may occur and be reported).
Anchored Sheet pile protected face – In conditions same as above but with larger depth of excavations and stringent control on the soil settlements/movement at the top of the wall, it is better to use a waler beam and anchors to support the wall at the top, also as shown in Fig.11.
In some cases, instead of using rolled interlocked steel sheet piles, the contractors also have used hollow sheet circular liner to retain the soil. In some of the cases, the passive anchors are not possible or permitted to be used (private property outside or a water body, etc.); in such cases instead of using prestressed anchors often the struts are used from outside, as shown in Fig. 12. In underground metro stations, this is a popular option.
The analysis of these problems can be done using software programs like Plaxis 2D, Wall-up and Midas NX-GTX, Geo-Studio, Deep-Ex etc. When the soil retention problem is more advanced in terms of loads, precision, depth, and time duration, then a permanent solution is required. Following methods of construction provides a permanent solution for the earth retention near the buildings (Fig 13).
- Continuous (touching) piled wall
- Secant pile wall.
- Diaphragm walls.
Instrumentation and Monitoring
In major and long duration projects (like underground metros or deep basements of buildings), during construction, the ground movement of adjoining structures may tilt, shift or settle and hence need to be monitored carefully. Tilto-meters, crack monitoring devices, gauges etc., need to be planned and carefully installed, maintained and monitored on a daily/weekly basis (as per the nature of the job). The stresses in the struts are also monitored and denoted as – (a) GREEN or Safe level, (b) BLUE Level (i.e. maintenance cycle to start), (c) YELLOW Level or Alert level (i.e. force exceeded the safe limit but within yield), (d)RED Level or Alarm level (force is reaching towards yield limit – prestressing to be done).
Summary and Conclusions
Failure of buildings due to construction of deep trench in the adjoining plot is uncommon and undesirable; however such failures have been witnessed in the past. A few past case studies are described which include (1) the 1990 excavation collapse at 14th and H Streets, N.W. Washington, D.C, USA, (2) the 2009 pile foundation failure and subsequent collapse of a G+13 building in Shanghai, China, (3) The 2018 landslide near a multistory building in Wadala, Mumbai and (4) the recent collapse of two buildings into adjacent trench in Bengaluru.
Treatments have to be done to the exposed vertical cut surfaces in order to avoid such failures. The type of treatment depends on (1) the type of strata (soil, silt, clay, gravel, rock, mixed strata), (2) depth of the excavation, (3) extent of pore water pressure, (4) extent of surface surcharge, i.e. weight of building, (5) seismic zone, (6) duration of exposed cut (7) distance of the cut from existing building foundations. The possible treatments include (1) leaving the bare exposed face as it is, in the case of shallow cuts, (2) Applying shotcrete to the exposed face of soil with or without wire mesh, (3) Providing soldier piles and timber planks lagging, (4) Providing cantilever sheet pile protected face, (5) providing anchored sheet pile protected face, (6) having continuous piled wall, (7) constructing a secant pile wall, and (8) building a diaphragm wall. In major long duration projects, it may be necessary to continuously monitor the ground movement and take appropriate action.
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- ASCE website and various newspapers feeds